Nitrous oxide emissions

Key points

  • Through the Climate Change Research Program, the Australian Government funded research to find practical and affordable ways to reduce emissions of nitrous oxide (N2O), a gas that has 310 times the global warming potential of carbon dioxide.
  • The majority of Australia’s N2O emissions come from agriculture, including fertilisers, livestock waste and burning crop stubble.
  • The Nitrous Oxide Research Program has found N2O emissions can range from 0.03 to 1kg  N/ha per day depending on environment and soil type.
  • Enhanced efficiency fertilisers, which are a combination of fertiliser and breakdown inhibitors, have been found to reduce N2O emissions across a range of soil types.

The Nitrous Oxide Research Program provided world class data on N2O emissions from different soils under different management practices, farming systems and climates. It identified N2O emission reduction strategies that may benefit farmers.

Adopting farming practices that lower emissions is likely to deliver economic and environmental benefits through lower fertiliser inputs, improved natural resource management and the potential to create offsets under the Carbon Farming Initiative.

How is nitrous oxide produced?

Nitrous oxide (N2O) emissions occur as a result of soil microbial activity. There are two chemical processes that produce N2O emissions:

  • Nitrification is an aerobic (oxygen present) process that converts ammonium (NH4) into nitrate (NO3) with N2O as a by–product
  • Denitrification is an anaerobic (oxygen absent) process that converts nitrate (NO3) into nitrogen gas (N2) with N2O being produced as an intermediate product.

Factors that influence nitrous oxide emissions

N2O emissions from agricultural soils are highly variable. For example, on a semi–arid property in the Western Australian wheat belt N2O emissions are less than 0.03kg N/ha per day, while in the high rainfall zone in the fertile pasture soils in south–eastern Victoria they are in excess of 1kg/ha per day.

The three main drivers of N2O emissions from Australian agricultural soils are soil carbon content, soil moisture content and nitrogen inputs.

Soil carbon content

The carbon (organic matter) content of a soil is a major driving factor in the amount of N2O it can emit. Farming systems that produce large amounts of carbon, either as pasture or crop residues, have the potential to emit higher levels of N2O. This is because the carbon provides energy to bacteria that carry out the denitrification process.

Preliminary research from the Nitrous Oxide Research Program has found that in some regions retaining crop residues can lead to high N2O emissions.

Soil moisture content

Soil moisture is an important driver of N2O emissions through both the nitrification and denitrification process. Nitrification mostly occurs when soils are at field capacity (when drainage has stopped). Denitrification occurs when soils are above field capacity and starting to become waterlogged.

The Nitrous Oxide Research Program research has demonstrated that N2O emissions can increase after high rainfall and irrigation events. Agricultural areas with high rainfall or irrigation applications tend to produce more organic matter which makes them conducive to emitting higher levels of N2O.

Nitrogen inputs

Farmers and land managers often apply fertiliser and animal manure to their soils to increase nitrogen levels and farm productivity. When these inputs exceed the amount of nitrogen that can be absorbed by plants they lead to increased N2O emissions.

The Nitrous Oxide Research Program found that high N2O emissions from a Victorian dairy farm were due to a combination of all three main drivers of N2O emissions. These drivers were:

  • a high and localised source of nitrogen from animal urine
  • high rainfall in the region
  • a high level of carbon from the pasture.

Reducing nitrous oxide emissions

The Nitrous Oxide Research Program explored different methods for reducing N2O emissions. These irrigation management, crop rotations, sowing techniques, liming and using enhanced efficiency fertilisers.

Irrigation management

Nitrous Oxide Research Program researchers found that N2O emissions from a winter grain/summer cotton farming system at Kingsthorpe, Queensland, were predominantly controlled by irrigation and rainfall events. In particular, the amount of irrigation water applied had a substantial influence on the intensity of emission events. The preliminary results suggest that the timing and amount of irrigation water applied to crops could play a role in reducing N2O emissions.

Crop rotations

Crop rotation trials examined the effectiveness of substituting fertiliser with legume crops or using non–leguminous crops to mop up excess nitrogen during fallow periods. Preliminary research indicates that rotating sugar cane with soy bean (a legume) can increase the nitrogen available to plants in the soil, reducing the overall need for nitrogenous fertiliser.

Sowing techniques

The Nitrous Oxide Research Program researchers found that there may be potentially higher N2O emissions resulting from conventional cultivation compared to direct drilling techniques.

Liming

Early research has found that applying lime to bare soils after significant summer/autumn rain can decrease N2O emissions if the soil was fertilised during winter cropping.

Enhanced efficiency fertilisers

Enhanced efficiency fertilisers (EEFs) are a combination of fertiliser and breakdown inhibitors. They can increase plant uptake of nitrogen and thereby reduce the loss of nutrients through leaching, runoff and as gas emissions. They work by delaying the chemical process that nitrogen compounds go through to produce ammonium and nitrate, both precursors to N2O. (See How is nitrous oxide produced?)

Nitrous Oxide Research Program researchers found that EEFs can work well across a range of soil types (see Snapshot). In some applications, the rate of N2O emissions can be reduced for up to 60 days.

Snapshot – Reducing N2O emissions from Australian intensive agricultural systems using enhanced efficiency fertilisers

A team of Nitrous Oxide Research Program researchers, led by Dr Deli Chen of the University of Melbourne, conducted a series of lab and field experiments examining the effectiveness of EEFs. Soils were sourced from a range of locations and production systems:

  • sugarcane (Mackay Qld)
  • cropping (Tamworth NSW)
  • cotton/cropping (Kingsthorpe Qld)
  • pasture seed (Murroon south western Vic).

EEFs are a combination of fertilisers and breakdown inhibitors. Testing was carried out on EEFs containing the following inhibitors:

  • The urease inhibitor (n–butyl) thiophosphoric triamide (NBPT) works by controlling the chemical process which converts urea to ammonia, a precursor to N2O.
  • The nitrification inhibitors dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD) work by controlling the chemical process which converts ammonium to nitrate.

Results:

Early results indicate that EEFs can reduce N2O emissions, with varying levels of success across different agricultural systems. For example, an EEF with the inhibitor DMPP was seen to reduce N2O emissions by 14 to 98 per cent across a range of soil types.

Results from a field trial conducted in a ryegrass pasture system in south–western Victoria show the application of EEF with the inhibitor DMPP reduced N2O emissions by 73 per cent when compared to urea application alone.

Results of a field trial conducted in ryegrass pasture at Murroon in south–west Victoria to determine the effect of an EEF containing the nitrification inhibitor DMPP on N2O emissions.

Partnerships

Research presented in this fact sheet is a result of partnerships between:

  • Dairy Australia
  • Department of Primary Industries Victoria
  • Grains Research and Development Corporation
  • Industry and Innovation New South Wales
  • Queensland University of Technology
  • Queensland Department of Environment and Resource Management
  • University of Melbourne
  • University of Western Australia

About the Climate Change Research Program

The Climate Change Research Program was part of Australia’s Farming Future, an Australian Government climate change initiative for primary industries. The Climate Change Research Program funded research projects and on–farm demonstrations to help prepare Australia’s primary industries for climate change. Research focused on reducing greenhouse gas pollution, improving soil management and climate change adaptation. The program provided practical management solutions to farmers and industries. The Department of Agriculture, Fisheries and Forestry managed the Climate Change Research Program.

Acknowledgement

Information contained in this fact sheet was obtained from a research progress report provided by Queensland University of Technology, including input from the research partners listed.